Okay, you can also disable some services, here's a link which describes what the service is and what it is recommended you set them to.
http://www.beemerworld.com/tips/servicesxp.htm Remember that you do need some of the services!!!!!
p.s if you disable a service and then reboot and your PC doesn't work or is worse, then you can restart the service or in a worst case scenario you can reboot into safe mode and restart the service.
Best bet is to do a trial and error, disable a service, write it down on a piece of paper, then reboot and try out the performance.
Some services are required to run and have Internet etc, others are loaded by XP just in case, even though you might never need them.
But as the people have said:
1. Remove programs you don't use.
2. Scan for SPYWARE
3. Scan for Viruses
4. Run MSCONFIG and stop any programs you don't want to start from loading
5. Clear out you Internet cache
6. If you aren't worried about restore points, turn it off (saves drive space)
7. Clear out temporary files
8. Use a program like REG-CLEAN to keep the registry tidy
9. Disable some of the services you don't use
10. Run a regular defrag (do it every few days when you are going to sleep)(some games like WOW cause a LOT of fragmentation of files)
p.s Remember not to have too many things running all at once, some people have games and browsers and chat programs and picture viewers all running at the same time, keep it to a minimum, and close some down as you don't use them etc.